Function Table #1

A function table is a table of order pairs that follow a rule. A rule tells how one number is related to another. In this brainteaser, I will give you a function table (sometimes called an input-output table) and you need to figure out what the rule is. I will give you an example.

Example:

Input(A), Output(B)
7(A), 9(B)
13(A), 15(B)
1(A), 3(B)
3(A), 5(B)
9(A), 11(B)
The rule in this example is add 2. Now here is the real deal. Find out what the rule is.
*This one will be harder than the example. Way harder.